Method and system for power efficient transmission of scalable video over wireless networks

ABSTRACT

A method and system for reducing power consumption in a wireless network by adjusting the transmit energy for each bit at the physical layer and the retry limit at the MAC layer. The method includes creating a look-up table containing optimal pairs of N lim ,E t  for a plurality of different sets of transmission properties, wherein N lim  is a retry limit and E t  is a transmit energy per bit; determing a set of transmission properties for a sequence of scalable video to be transmitted over the wireless network; accessing the look-up table to obtain the optimal pair of N lim ,E t , corresponding to the set of determined transmission properties; and transmitting the sequence of scalable video over the wireless network using the accessed optimal pair of N lim ,E t .

The present invention relates in general to wireless networks, and moreparticularly, to a method and system for power efficient transmission ofscalable video over wireless networks (e.g., in a wireless networkincluding portable multimedia devices).

Because of the high throughput provided by wireless local area networks(WLAN), real-time video communication over a WLAN is becoming feasible.

The possible applications include video communications on portabledevices, portable video servers, etc. These types of WLAN devices oftenrely on batteries for operation. Batteries have limited life time andfrequent recharging is not desirable. With the integration of videotransmission, which requires high bandwidth and high power fortransmission, power management becomes even more important.

The present invention considers the situation where scalable video datais transmitted over a WLAN, in which retransmission is adopted as theerror control scheme. One goal of the present invention is to keep aconstant video quality at the receiver while minimizing the overalltransmission power, or conversely, to optimize video quality given afixed transmission power resource.

In the present invention, to minimize the transmission power whilekeeping a constant video quality at the receiver, the transmissionenergy at the physical layer and the retransmission scheme at the mediumaccess control (MAC) layer are considered. In particular, the presentinvention reduces power consumption by adjusting the transmit energy foreach bit at the physical layer and the retry limit at the MAC layer.

In general, the present invention provides a method for power efficienttransmission of scalable video over a wireless network, comprising:creating a look-up table containing optimal pairs of N_(lim),E_(t) for aplurality of different sets of transmission properties, wherein N_(lim)is a retry limit and E_(t) is a transmit energy per bit; determining aset of transmission properties for a sequence of scalable video to betransmitted over the wireless network; accessing the look-up table toobtain the optimal pair of N_(lim),E_(t) corresponding to the set ofdetermined transmission properties; and transmitting the sequence ofscalable video over the wireless network using the accessed optimal pairof N_(lim),E_(t).

The present invention also provides a system for power efficienttransmission of scalable video over a wireless network, comprising: alook-up table containing optimal pairs of N_(lim),E_(t) for a pluralityof different sets of transmission properties, wherein N_(lim) is a retrylimit and E_(t) is a transmit energy per bit; a system for determining aset of transmission properties for a sequence of scalable video to betransmitted over the wireless network, and for accessing the look-uptable to obtain the optimal pair of N_(lim),E_(t) corresponding to theset of determined transmission properties; and a system for transmittingthe sequence of scalable video over the wireless network using theaccessed optimal pair of N_(lim),E_(t).

The present invention further provides a program product stored on arecordable medium for providing power efficient transmission of scalablevideo over a wireless network, comprising: program code for determininga set of transmission properties for a sequence of scalable video to betransmitted over the wireless network; and program code for accessing alook-up table containing optimal pairs of N_(lim),E_(t) for a pluralityof different sets of transmission properties, wherein N_(lim) is a retrylimit and E_(t) is a transmit energy per bit, to obtain the optimal pairof N_(lim),E_(t) corresponding to the set of determined transmissionproperties, wherein the sequence of scalable video is transmitted overthe wireless network using the accessed optimal pair of N_(lim),E_(t).

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings in which:

FIG. 1 illustrates the effect of the maximum retry limit N_(lim) on theoverall transmission power.

FIG. 2 illustrates a fine-granular-scalable video transmission system.

FIG. 3 illustrates PSNR for a sample video sequence.

FIG. 4 illustrates transmission for different retry limits.

FIGS. 5A-5C illustrate, for a large p_(L)=1%, the required transmissionenergy per bit E_(t) required for a given PSNR; the average number oftransmissions needed to transmit one packet successfully; and thetransmission energy per bit to transmit one bit successfully, includingretransmission.

FIGS. 6A-6C illustrate, for a small p_(L)=0.01%, the requiredtransmission energy per bit E_(t) required for a given PSNR; the averagenumber of transmissions needed to transmit one packet successfully; andthe transmission energy per bit to transmit one bit successfully,including retransmission.

FIG. 7 illustrates power consumption versus distance.

FIG. 8 illustrates a flowchart in accordance with an embodiment of thepresent invention.

FIG. 9 illustrates a transmission system in accordance with anembodiment of the present invention.

FIG. 10 illustrates a computer system for implementing the power managerof the present invention.

It should be noted that the drawings are merely schematicrepresentations, not intended to portray specific parameters of theinvention. The drawings are intended to depict only typical aspects ofthe invention, and therefore should not be considered as limiting thescope of the invention.

The present invention describes a method and system for reducingtransmission power consumption for scalable video communication over awireless network (e.g., a WLAN). This is achieved by choosing themaximum number of retransmission times based on the quality and delayrequirement. Transmitter SNR is adjusted accordingly to maintain aconstant end-to-end video quality. For different retry limits, hencedifferent transmitter SNR's, we get different power consumptions. Thepresent invention finds and uses the transmission power level tominimize the overall energy for power efficient scalable videotransmission. Using the present invention, power efficient transmissionof scalable video over a wireless LAN is achieved by adjusting theretransmission limit and the transmission power level given theunderlying channel condition (SNR), which can be affected by noise,interference, and distance between the transmitter and the receiver.

Referring now to FIGS. 1(a)-(d), there is illustrated the effect of themaximum retry limit N_(lim) on the overall transmission power. As shown,as the retry limit increases, to keep the same end-to-end video quality,(1) the video stream can tolerate higher packet loss rate; (2) thetransmit energy per bit is lower (3) the number of retransmission timesincrease or stay the same; (4) overall transmission power varies.

Generally, the larger the retry limit at the MAC layer is, the moreretransmission numbers the transmitter can have. Hence, the number oftransmissions, which includes both the first transmission and thefollowing retransmission(s), is an increasing function of the retrylimit N_(lim) as illustrated in FIG. 1(a). On the other hand, as thenumber of transmissions increases, the error control capability isenhanced, so that to keep a given quality at the receiver, the packetloss rate before any retransmission can be higher, i.e., the stream cantolerate more errors introduced by transmission. The relationshipbetween the packet loss rate and retry limit is shown in FIG. 1(b). Itis clear that the transmission energy per packet is a decreasingfunction of the packet loss rate. Thus, it is a decreasing function ofretry limit N_(lim) as shown in FIG. 1(c). Integrating 1(a) and (c), theoverall power as a function of the transmission energy and the number oftransmission has an optimal point N*_(lim) (FIG. 1(d)) that minimizesthe power consumption, hence prolonging battery life.

As an illustrative example, a Fine-Granular-Scalable (FGS) encoded videostream is to be transmitted. At the MAC layer, the retry limit can beadapted to the video quality requirement and the underlying channelconditions. At the physical layer, the energy to transmit a bit can beadjusted. In the following analysis, it is assumed that the bit streamis transmitted over an additive white Gaussian noise (AWGN) channel, andthe theoretical effect of retry limit and transmission energy on theoverall transmission power is analyzed. Then, a numerical analysis ispresented for selecting the optimal operating point to minimize theoverall power consumption.

Analytical Model

The system 10 considered in the present invention is illustrated in FIG.2. In system 10, the video stream is compressed by aFine-Granular-Scalable (FGS) encoder 12, modulated using differentialphase-shift-keying, and transmitted over an underlying additive whiteGaussian noise (AWGN) channel 14. The retry limit at the MAC layer 16,and the transmit energy at the physical layer 18, are adjusted to thevideo quality requirement and the underlying channel conditions. Nochannel encoder is used above the MAC layer.

The distortion caused by the FGS encoder 12 is first described. Theparameters and the power consumption leading to a given distortion atthe receiver by adjusting the MAC layer 16 and the physical layer 18 arediscussed in the following section.

Distortion Model of the FGS Video Encoder

FGS encoding provides smooth quality degradation in order to adapt tochanging network conditions. The FGS encoder 12 includes a base-layerencoder 20 and the enhancement layer encoder 22. The base layer iscompressed by the base-layer encoder 20 using motion-compensationencoding method; the enhancement-layer encoder 22 is based on afine-granular coding method. In this discussion, it is assumed that allbase-layer bits are received without any error. The enhancement layerdata is organized into packets and sent through the unreliable channel.

PSNR-rate Performance of the FGS Encoder

The FGS encoder 12 provides an almost linear relationship between theenhancement layer bit rate and the peak signal to noise ratio (PSNR), asshown in FIG. 3 for a sample video sequence. The linear function can bewritten as followsPSNR=k _(FGS) R _(s) +c _(FGS)   (1)

where R_(s) is the encoded source bit rate, and PSNR is thecorresponding video PSNR. In FIG. 3, the parameters are derived by theleast-mean-square-error method. For the sample video sequence, encodedat a frame rate of 30 fps and quantization stepsize of 10, the parametervalues are listed in Table 1. From FIG. 3, it can be seen that there isa good match between the measurement data and the linear model. TABLE 1Simulation parameter settings Parameter Type Parameter Value PacketsizeM (bytes) 1000 k_(FGS) (dB/Mbps) 1.66 c_(FGS) (dB) 30.29 R (Mbps) 2.84R_(bl) (Mbps) 0.67Average PSNR at the Receiver at the Presence of Packet Loss

In the previous section, it was shown that the reconstructed videoquality, measured in PSNR, is a linear function of encoded bit stream ifno errors occur during transmission. The PSNR of the decoded videosequence when transmission error is introduced will now be presented,given a video sequence of a frame rate of fr fps, where each encodedframe consists of base layer data and N_(el) packets of enhancementlayer data. It is assumed that when one packet error occurs, allfollowing enhancement layer packets corresponding to the same videoflame are discarded. When the first i packets are correctly received forone frame, the corresponding data rate at the receiver R_(i) isR _(i) =fr×i×M+R _(bl)   (2)where M is the packet size, and R_(bl) is the data rate for the baselayer. The parameter values used for numerical analysis in thisdisclosure are listed in Table 1. At the receiver, the video PSNR willbePSNR _(i) =k _(FGS) R _(i) +c _(FGS)   (3)The average PSNR at the receiver is $\begin{matrix}\begin{matrix}{{PSNR} = {\sum\limits_{t = 0}^{N_{el}}{p_{i}{PSNR}_{i}}}} \\{= {\sum\limits_{i = 0}^{N_{el}}{p_{i}\left( {{k_{FGS}R_{i}} + c_{FGS}} \right)}}} \\{= {{k_{FGS}{\sum\limits_{i = 0}^{N_{el}}{p_{i}R_{i}}}} + c_{FGS}}}\end{matrix} & (4)\end{matrix}$where p_(i) is the probability that the first i packets are receivedsuccessfully.

Defining those data kept by the receiver as the effective data, and theamount of the effective data in one second as effective data rateR_(el), which can be calculated as follows, $\begin{matrix}{R_{el} = {\sum\limits_{i = 0}^{N_{el}}{p_{i}R_{i}}}} & (5) \\{{we}\quad{get}} & \quad \\{{PSNR} = {{k_{FGS}R_{el}} + c_{FGS}}} & (6)\end{matrix}$Hence, as long as we can get a data rate of R_(el), we expect thereceiver can reach the corresponding PSNR on average.

If the residual packet loss rate after retransmission is p_(L),$\begin{matrix}{p_{i} = \left\{ \begin{matrix}{{\left( {1 - p_{L}} \right)^{i}p_{L}},} & {{{{for}\quad i} = 0},1,\ldots\quad,{N_{el} - 1}} \\{\left( {1 - p_{L}} \right)^{N_{el}},} & {{{for}\quad i} = N_{el}}\end{matrix} \right.} & (7)\end{matrix}$Combining Eq. (2), (5) and (7), R_(el) can be written as following,$\begin{matrix}\begin{matrix}{R_{el} = {{\sum\limits_{i = 0}^{N_{el} - 1}{\left\lbrack {{\left( {1 - p_{L}} \right)^{t}p_{L}i} + {\left( {1 - p_{L}} \right)^{N_{el}}N_{el}}} \right\rbrack{Mfr}}} + R_{bl}}} \\{= {{\left\lbrack {\frac{1 - p_{L}}{p_{L}} - \frac{\left( {1 - p_{L}} \right)^{N^{el} + 1}}{p_{L}}} \right\rbrack{Mfr}} + R_{bl}}}\end{matrix} & (8)\end{matrix}$

For a particular algorithm, in which N_(el) and M are fixed, R_(el) isdetermined by p_(L). Thus, the average PSNR at the receiving side isdecided by the residual packet error rate p_(L). In the followingsection, it will shown how p_(L) is related to the retry limit at theMAC layer, transmit SNR at the physical layer, and power consumption.

Transmission Model

It will be shown how the retry limit N_(lim) and the transmit SNR at thephysical layer control p_(L), i.e., the PSNR at the receiver. Theinformation bit stream is organized into packets, each containing Minformation bits. Packet error occurs when the receiver detects there iserror within the received packet (even one single bit error can cause apacket error). The probability that a packet is erroneous, p_(p0),depends on the received signal to noise ratio per bit. The physicallayer will be discussed first, where the bit error rate is determined bythe channel characteristics and the transmit energy E_(t) applied toeach bit. Then, the manner by which the retransmission will reduce theerror at the receiving side and how it introduces extra energyconsumption by using multiple transmission for one video packet will bediscussed.

Packet Error Rate p_(p0)

For simplicity, it is assumed that the channel is an AWGN channel, DPSKis used for modulation, and E_(t) is the transmit energy per bit. Thereceived energy per bit E_(b) at the receiver is proportional to h,i.e., E_(b)=hE_(t), the path gain between two mobiles, which depends onthe distance between them, where h is given byh=cd^(−α)  (9)where c is a constant, and d is the distance between two stations. Inthis example, α=3.6. The value of c is chosen such that when twoterminals are 100 m away, the received SNR per bit is from 2 dB to 16dB. The bit error rate (BER) is $\begin{matrix}{{p_{b}\left( E_{b} \right)} = {\frac{1}{2}{\mathbb{e}}^{\frac{E_{b}}{N_{0}}}}} & (10)\end{matrix}$where N₀ is noise power spectral density.

The packet error occurs when there is even one single bit error. For apacket of M bits, the packet error rate is $\begin{matrix}{p_{p\quad 0} = {{1 - {\left( {1 - {p_{b}\left( E_{b} \right)}} \right)M}} = {1 - \left( {1 - {\frac{1}{2}{\mathbb{e}}^{\frac{E_{b}}{N_{0}}}}} \right)}}} & (11)\end{matrix}$Residual Packet Loss Rate p_(L)

In wireless LAN, retransmission is used as the error control scheme.Only when all N_(lim)+1 transmissions are erroneous, will a packet notget through the channel successfully. Hence, the residual packet errorrate, which is the probability that a packet is erroneous afterN_(lim)+1 transmissions, isp_(L)=p_(p0) ^(N) _(lim) ⁺¹   (12)As shown in FIG. 4, if the retry limit is higher, more packets may betransmitted correctly.Average Transmission Times N_(tr)

As shown in FIG. 4, each video packet is transmitted until it issuccessfully transmitted or reaches the retry limit. The probabilitythat a video packet is successfully sent at n^(th) try is p_(p0)^(n−1)(1−p_(p0)), while the probability that the transmission of a videopacket reaches the retry limit without being successfully sent is p_(p0)^(N) _(lim) ⁺¹. Overall, the average number of transmissions for a videopacket is $\begin{matrix}{{N_{tr}\left( N_{\lim} \right)} = {{\sum\limits_{n = 1}^{N_{\lim} + 1}{{np}_{p\quad 0}^{n - 1}\left( {1 - p_{p\quad 0}} \right)}} + {\left( {N_{\lim} + 1} \right)p_{p\quad 0}^{N_{\lim} + 1}}}} & (12)\end{matrix}$Overall Transmission Energy

From Eq. (12), it can be seen that for one video packet, eithersuccessfully transmitted or discarded due to limit on MAC layer retry,the average energy used isE _(p)(E _(t) ,N _(lim))=E _(t) ×N _(tr)(N _(lim))×M   (13)

For a video sequence of a frame rate of fr fps, and for each framecontaining N_(el) packets in the enhancement layer, the powerconsumption by the enhancement layer data isP _(all)(E _(t) ,N _(lim))=E _(t) ×N _(tr)(N _(lim))×M×N _(el) ×fr  (14)The optimization problem of the present invention can therefore beformulated as

-   -   Min P_(all) subject to delay and bandwidth constraint.

In this example it is assumed that there is a sufficiently highbandwidth. If the upper bound of the retry limit is set to satisfy thedelay constraint, the optimization problem can be specified asMin P _(all)(E _(t) ,N _(lim))=E _(t) ×N _(tr)(N _(lim))×M×N _(el) ×frSubject to N_(lim)<N_(upper).Numerical Results

In this section, the performance of the method of the present inventionis examined. First, the performance under different quality requirementswhen the distance between the transmitter and the receiver is fixed isconsidered. Then, the case where the quality requirement is the same,but the receiver is moving around, is considered. The parameter valuesused in the simulation are summarized in Table 1. Here the sample videosequence is encoded at a frame rate of 30 fps, and the transmitted datarate is 2.84 Mbps, corresponding to a PSNR of 35 dB if no error occurs.The base layer data rate is 0.67 Mbps and the PSNR reconstructed fromthe base layer is 30.29 dB. The enhancement layer data is packetizedinto 9 packets, each containing 1000 bytes. At the physical layer, thereceived signal to noise ratio is chosen from 2 dB to 16 dB when twomobiles are 10 m away. The maximum retry limit is set at N_(upper)=20 soas to guarantee one packet can be received within the delay constraint.

For the figures (i.e., FIGS. 5A-C, 6A-C) demonstrating the performance,the power consumption is normalized by a scaling factor $\begin{matrix}{c_{sf} = {\frac{1}{c}d_{0}^{\alpha}N_{0}{MN}_{el}{fr}}} & (18)\end{matrix}$i.e., the values shown in the figures are $\begin{matrix}{P_{sf} = {\frac{E_{b}}{N_{0}}\left( \frac{\mathbb{d}}{\mathbb{d}_{0}} \right)^{\alpha}N_{tr}}} & (19)\end{matrix}$Minimize for Different Requirement of PSNRs

In this section, the manner by which the optimal points vary with therequirement of PSNR when the distance is fixed at d=10 m is analyzed.Different PSNR requirements at the receiving side will be considered.Two different p_(L) are used to simulate different quality requirements.The results are presented in FIGS. 5A-C and 6A-C.

The transmission energy per bit E_(t) for a given PSNR, the averagenumber of transmissions needed to transmit one packet successfully, andthe transmission energy per bit to transmit one bit successfully,including retransmission, as described in Eq. (14), (15) and (17) for agiven video quality corresponding to p_(L)=1%, are illustrated in FIGS.5A-C, respectively. As shown in FIG. 5A, as the retry limit increases,the same video quality can be obtained by a lower energy per bit E_(t).Also, as shown in FIG. 5B, as the retry limit increases, there may bemore retransmissions deployed in the presence of severe channelimpairment.

Combining FIGS. 5A and 5B, the power consumption is shown in FIG. 5C.The power consumption is scaled by c_(sf) as in Eq. (18). The optimalpoint OP here occurs at N_(lim)=1, i.e., increasing transmission energyper bit E_(t) is always more efficient than transmitting more times forhigh p_(L). Comparing the power consumption at the optimal point withthe power consumption at N_(lim)=10, a power saving of around 50% can begained.

In FIGS. 6A-C, the scenario for p_(L)=0.01%, representing a higherreceiving video quality for the same channel condition in FIGS. 5A-C, isillustrated. Comparing FIGS. 6A-C with FIGS. 5A-C, it can be seen thatfor higher quality, a greater retry limit to gain high error correctioncapability is needed. In particular, as shown in FIG. 6C, forp_(L)=0.01%, the optimal point occurs with retransmission limitN*_(lim)=3.

Minimize the Power Consumption Over a Range of Distance

In this section, the scenario when the receiving terminal moves aroundis analyzed (e.g., a distance from 10 m to 20 m, simulating a homeenvironment). For each distance, the optimal pair of (N_(lim),E_(t)) iscalculated. The results are summarized in FIG. 7. Note here that E_(t)is normalized with respect to $\frac{c}{d_{0}^{\alpha}N_{0}},$where d₀=10 m. It has been found that when the distance goes large, highretry limits are preferred. The power consumption for N_(lim)=10 is alsoshown in FIG. 7. Comparing the two curves, the algorithm of the presentinvention outperforms the scheme in which N_(lim) is set to a largevalue. If N_(lim) is set to a small value, for instance, N_(lim)=1, itrepeats the optimal curve for the small distance up to d=16 m in thissimulation, but it is unable to adapt to the large distance, i.e., thequality can not be kept at the desired level. There is a fluctuation forthe N_(lim)=10 scheme and some inconsistency for optimal curve. This isdue to the fact that for discrete sets of N_(lim) and E_(t), theresulted PSNR is in fact not a constant, but always higher than theexpected value.Implementation

The 802.11 MAC/PHY standard allows devices to alter the transmissionenergy level and retry limit on the fly. Both increasing retry limitsN_(lim) at the MAC layer and the transmission energy level E_(t) at thephysical layer (PHY) provide higher error protection for the datatransmitted. However, to reach the same video quality at the receiver,they act differently in the sense of power consumption. The presentinvention determines the optimal pair of (N_(lim),E_(t)) that minimizesthe power consumption.

A flowchart 100 and system diagram 200 illustrating an implementation ofthe present invention are provided in FIGS. 8 and 9, respectively. Thisimplementation provides a “power manager 102,” whose operations may bedistributed between a base station B and one or more portable terminalsTER over a wireless network.

In step S1, the adaptation rules for a discrete set of qualityrequirements, channel 114 conditions, and video sequence properties(e.g., the relationship between PSNR and the rate for FGS encoder 112)are pre-computed and stored as a look-up table 104 in the base stationB. An optimal operating pair of (N_(lim),E_(t)) is provided in thelook-up table 104 for each set of data.

In step S2, during communication of a scalable video sequence, the QoSrequirements, channel conditions, and video sequence properties aredetected and are reported to the power manager 102. Based on thiscriterion, the power manager 102 determines the optimal operating pairof (N_(lim),E_(t)) by accessing the pre-computed look-up table 104. TheN_(lim) from the optimal operating pair is provided to the MAC layer116, while the E_(t) from the optimal operating pair is provided to thePHY layer 118. In step S3, these operating points are updated frequently(e.g., after time T) in order to follow the time-varying, applicationspecific characteristics of the wireless channel 114.

It should be understood that the present invention can be realized inhardware, software, or a combination of hardware and software. Any kindof computer/server system(s)—or other apparatus adapted for carrying outthe methods described herein—is suitable for the practice of the presentinvention. A typical combination of hardware and software could be ageneral purpose computer system with a computer program that, whenloaded and executed, carries out the respective methods describedherein. Alternatively, a specific use computer, containing specializedhardware for carrying out one or more of the functional tasks of theinvention, could be utilized. The present invention can also be embeddedin a computer program product, which comprises all the respectivefeatures enabling the implementation of the methods described herein,and which—when loaded in a computer system—is able to carry out thesemethods. Computer program, software program, program, or software, inthe present context mean any expression, in any language, code ornotation, of a set of instructions intended to cause a system having aninformation processing capability to perform a particular functioneither directly or after either or both of the following: (a) conversionto another language, code or notation; and/or (b) reproduction in adifferent material form.

An example of a computer system 300 is shown in FIG. 10. The computersystem 300 generally comprises a central processing unit (CPU) 302,memory 304, input/output (I/O) interfaces 306, bus 308, external devices310 and database 312. A user 314 may interact with the computer system300 (e.g., to generate look-up table 104 (FIG. 9)).

Computer 300 can comprise any general purpose or specific-use systemutilizing standard operating system software, which is designed to drivethe operation of the particular hardware and which is compatible withother system components and I/O controllers. The CPU 302 may comprise asingle processing unit, multiple processing units capable of paralleloperation, or can be distributed across one or more processing units inone or more locations, e.g., on a client and server. The memory 304 maycomprise any known type of data storage and/or transmission media,including magnetic media, optical media, random access memory (RA), etc.Moreover, similar to the CPU 302, the memory 304 may reside at a singlephysical location, comprising one or more types of data storage, or bedistributed across a plurality of physical systems in various forms.

The I/O interfaces 306 may comprise any known system for exchanginginformation with one or more external devices 310. The external devices310 may comprise any known type of input/output device capable ofcommunicating with I/O interfaces 306 with or without additionaldevices. The bus 308 provides a communication link between each of thecomponents in computer 300 and likewise may comprise any known type oftransmission link, including electrical, optical, wireless, etc. Otherknown components may also be incorporated into the computer 300.

The database 312 may provide storage for information necessary to carryout the present invention. For example, the look-up table 104 (FIG. 9)may be stored within the database 312. The database 312 may include oneor more storage devices, such as a magnetic disk drive or an opticaldisk drive. Further, the database 312 can include data distributedacross a network such as LAN, WAN, or the Internet.

A power manager 320 in accordance with the present invention is shownstored in memory 304 as computer program code. The power manager 320includes a information system 322 for determining/receiving“transmission properties” such as QoS requirements, channel conditions,video sequence properties, etc., and an optimizing system 324 fordetermining the optimal operating pair of (N_(lim),E_(t)) for each timeT by accessing the pre-computed look-up table stored in the database312. N_(lim) and E_(t) are subsequently provided to the MAC and PYSlayers 116, 118 (FIG. 9) via the I/O interfaces 306.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to aperson skilled in the art are intended to be included within the scopeof the invention as defined by the accompanying claims.

1. A method for power efficient transmission of scalable video over awireless network, comprising: creating a look-up table containingoptimal pairs of N_(lim),E_(t) for a plurality of different sets oftransmission properties, wherein N_(lim) is a retry limit and E_(t) is atransmit energy per bit; determining a set of transmission propertiesfor a sequence of scalable video to be transmitted over the wirelessnetwork; accessing the look-up table to obtain the optimal pair ofN_(lim),E_(t) corresponding to the set of determined transmissionproperties; and transmitting the sequence of scalable video over thewireless network using the accessed optimal pair of N_(lim),E_(t). 2.The method of claim 1, wherein each set of transmission propertiesincludes at least one of quality-of-service requirements, channelconditions, and video sequence properties.
 3. The method of claim 1,further including: encoding the sequence of scalable video using aFine-Granular-Scalable encoder.
 4. The method of claim 1, wherein thesequence of scalable video is encoded using a base layer encoder and anenhancement layer encoder.
 5. The method of claim 1, further comprising:providing N_(lim) to a medium access control (MAC) layer of the wirelessnetwork; and providing E_(t) to a physical layer of the wirelessnetwork.
 6. The method of claim 1, further comprising: repeating thedetermining, accessing, and transmitting steps to follow time varyingcharacteristics of the wireless network.
 7. A system for power efficienttransmission of scalable video over a wireless network, comprising: alook-up table containing optimal pairs of N_(lim),E_(t) for a pluralityof different sets of transmission properties, wherein N_(lim) is a retrylimit and E_(t) is a transmit energy per bit; a system for determining aset of transmission properties for a sequence of scalable video to betransmitted over the wireless network, and for accessing the look-uptable to obtain the optimal pair of N_(lim),E_(t) corresponding to theset of determined transmission properties; and a system for transmittingthe sequence of scalable video over the wireless network using theaccessed optimal pair of N_(lim),E_(t).
 8. The system of claim 7,wherein each set of transmission properties includes at least one ofquality-of-service requirements, channel conditions, and video sequenceproperties.
 9. The system of claim 7, further including: aFine-Granular-Scalable encoder for encoding the sequence of scalablevideo.
 10. The system of claim 7, further including a base layer encoderand an enhancement layer encoder for encoding the sequence of scalablevideo.
 11. The system of claim 7, wherein the wireless network includesa medium access control (MAC) layer and a physical layer, and whereinN_(lim) is provided to the MAC layer of the wireless network and E_(t)is provided to the physical layer of the wireless network.
 12. Thesystem of claim 7, wherein the determining system updates the set oftransmission properties after a predetermined interval, and accesses thelook-up table to obtain an updated optimal pair of N_(lim),E_(t)corresponding to the updated set of determined transmission properties,and wherein the transmitting system transmits the sequence of scalablevideo over the wireless network using the updated optimal pair ofN_(lim),E_(t).
 13. A program product stored on a recordable medium forproviding power efficient transmission of scalable video over a wirelessnetwork, comprising: program code for determining a set of transmissionproperties for a sequence of scalable video to be transmitted over thewireless network; and program code for accessing a look-up tablecontaining optimal pairs of N_(lim),E_(t) for a plurality of differentsets of transmission properties, wherein N_(lim) is a retry limit andE_(t) is a transmit energy per bit, to obtain the optimal pair ofN_(lim),E_(t) corresponding to the set of determined transmissionproperties, wherein the sequence of scalable video is transmitted overthe wireless network using the accessed optimal pair of N_(lim),E_(t).14. The program product of claim 13, wherein each set of transmissionproperties includes at least one of quality-of-service requirements,channel conditions, and video sequence properties.
 15. The programproduct of claim 13, wherein the sequence of scalable video is encodedusing a Fine-Granular-Scalable encoder.
 16. The program product of claim13, wherein the sequence of scalable video is encoded using a base layerencoder and an enhancement layer encoder.
 17. The program product ofclaim 13, further comprising: program code for providing N_(lim), to amedium access control (WAC) layer of the wireless network, and programcode for providing E_(t) to a physical layer of the wireless network.18. The program product of claim 13, further comprising: program codefor updating the set of transmission properties after a predeterminedinterval, and for accessing the look-up table to obtain an updatedoptimal pair of N_(lim),E_(t) corresponding to the updated set ofdetermined transmission properties, and wherein the sequence of scalablevideo is transmitted over the wireless network using the updated optimalpair of N_(lim),E_(t).